1. Chapter 6: Functions Starting Out with C++ Early Objects Ninth Edition
by Tony Gaddis, Judy Walters, and Godfrey Muganda

2. Topics
6.1 Modular Programming 6.2 Defining and Calling Functions 6.3 Function Prototypes 6.4 Sending Data into a Function 6.5 Passing Data by Value 6.6 The return Statement 6.7 Returning a Value from a Function 6.13 Using Reference Variables as Parameters

3. 6.1 Modular Programming
Modular programming: breaking a program up into smaller, manageable functions or modules
Motivation for modular programming
Improves maintainability of programs
Simplifies the process of writing programs
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4. 6.2 Declaring, Defining and Calling Functions
Function Declaration (or prototype): declares a function before its use
Function definition: statements that make up a function
Function call: statement that causes a function to execute
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5. Function Declaration Declaration includes
return type: data type of the value the function returns to the part of the program that called it
name: name of the function. Function names follow same rules as variable names
parameter list: variables that hold the values passed to the function
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6. 6.3 Function Prototypes (Declarations)
Ways to notify the compiler about a function before a call to the function
Place function definition before calling function’s definition
Use a function prototype (similar to the heading of the function
Prototype: void printHeading();
See pr6-05.cpp
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7. Prototype Notes
Place prototypes near top of program (after constants and before main)
Program should include prototype before any call to the function, otherwise a compiler error occurs
Function definitions can be placed in any order in the source file (Traditionally main is placed first)
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8. Function Return Type
If a function returns a value, the type of the value must be indicated
int sumNumbers(int n, int [] nums);
If a function does not return a value, its return type is void
void printHeading();
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9. Function Documentation
Function should begin with a comment giving the function name and its purpose
Other useful comments include ones that indicate
What it does
Values that it expects, if any
Values that it returns, if any
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10. Example //Purpose: sums a list of integer numbers
//Requires: nums, array of ints
// n, the number of ints in nums
//Returns: an int, the sum of the numbers
int sumNumbers(int n, int [] nums);
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11. Example //Purpose: enter new student name in list of students
//Requires: students, array of strings
// n, the number of strings in students, n < MAX
// newStudent, string, newStudent not already
// in array
//Returns: n = old n + 1 and students with newStudent // // inserted at end of the array
int enterNewStudent(int & n, string[] & students, string newStudent);
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12. Function Definition Definition includes heading:
return type: data type of the value the function returns to the part of the program that called it
name: name of the function. Function names follow same rules as variable names
parameter list: variables that hold the values passed to the function
body: statements that perform the function’s task
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13. Function Definition

14. 6.4 Sending Data into a Function
Can pass values into a function at time of call
c = sqrt(a*a + b*b);
Values passed to function are arguments
Variables in function that hold values passed as arguments are parameters (also called formal arguments)
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15. Parameters, Prototypes, and Function Headings
For each function argument,
the prototype must include the data type of each parameter in its ()
void evenOrOdd(int num); //prototype
the heading must include a declaration, with variable type and name, for each parameter in its ()
void evenOrOdd(int num) //heading
The function call for the above function would look like this: evenOrOdd(val); //call
See pr6-06.cpp and pr6-07.cpp
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16. 6. 4 Calling a Function
To call a function, use the function name followed by () and ;
printHeading();
When a function is called, the program executes the body of the function
After the function terminates, execution resumes in the calling function at the point of call
Related, sample programs from the text are noted.
See pr6-01.cpp and pr6-02.cpp
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17. Calling a Function main can call any number of functions
Functions can call other functions
The compiler must know the following about a function before it is called
name
return type
number of parameters
data type of each parameter
See pr6-03.cpp and pr6-04.cpp
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18. Function Call Notes
Value of argument is copied into parameter when the function is called
Function can have > 1 parameter
There must be a data type listed in the prototype () and an argument declaration in the function heading () for each parameter
Arguments will be promoted/demoted as necessary to match parameters
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19. Calling Functions with Multiple Arguments
When calling a function with multiple arguments
the number of arguments in the call must match the function prototype and definition
the first argument will be copied into the first parameter, the second argument into the second parameter, etc.
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20. Calling Functions with Multiple Arguments Illustration
displayData(height, weight); // call void displayData(int h, int w)// heading { cout << “Height = “ << h << endl; cout << “Weight = “ << w << endl; }
See pr6-08.cpp
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21. 6.5 Passing Data by Value
Pass by value: when argument is passed to a function, a copy of its value is placed in the parameter
Parameter cannot access the original argument
Changes to the parameter in the function do not affect the value of the argument back in the calling function
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22. 6.6 The return Statement Used to end execution of a function
Can be placed anywhere in a function
Any statements that follow the return statement will not be executed
Can be used to prevent abnormal termination of program
Without a return statement, the function ends at its last }
See pr6-10.cpp
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23. 6.7 Returning a Value From a Function
return statement can be used to return a value from the function to the module that made the function call
Prototype and definition must indicate data type of return value (not void)
Calling function should use return value
assign it to a variable
send it to cout
use it in an arithmetic computation
use it in a relational expression
See pr6-11.cpp and pr6-12.cpp
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24. 6.8 Boolean return Example
bool isValid(int); // prototype bool isValid(int val) // heading { int min = 0, max = 100; if (val >= min && val <= max) return true; else return false; } if (isValid(score)) // call …
See pr6-13.cpp
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25. 6.13 Using Reference Variables
A reference variable is an alias for another variable
Defined with an ampersand (&)
void getDimensions(int&, int&);
Changes to a reference variable are made to the variable it refers to
Use reference variables to implement passing parameters by reference
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26. Pass by Reference Example
void squareIt(int &); //prototype void squareIt(int &num) { num *= num; } int localVar = 5; squareIt(localVar); // localVar’s // value is now 25
See pr6-24.cpp and pr6-25.cpp
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27. Reference Variable Notes
Each reference parameter must contain &
Argument passed to reference parameter must be a variable (cannot be an expression or constant)
Use only when appropriate, such as when the function must input or change the value of the argument passed to it
Files (i.e., file stream objects) should be passed by reference
See pr6-26.cpp and pr6-27.cpp
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